In recent years, there has been a great deal of interest in the development of alternative energy sources, or energy carriers, such as hydrogen. Automobiles and other vehicles that use hydrogen as a fuel source have been developed, and methods for refueling these vehicles that can compete with gasoline fuelling stations on scale and/or cost have been designed and are being further developed.
Nowadays, there is a range of possible supply chains for hydrogen to refueling stations, including delivery routes (e.g. truck or pipeline) and forecourt production routes (e.g. hydrocarbon reforming or water electrolysis). Hydrogen of a purity high enough for fuel cell cars and up to the relevant standards is delivered (in gaseous form, commonly) to one or more storage tanks at the station, then compressed and stored in high pressure buffer tanks. It is then cooled and dispensed. A single dispenser may have more than one hose, providing hydrogen of different purities or at a different pressure. Depending on the source of the hydrogen, an initial purification step at the refueling station may be necessary in order to produce a hydrogen stream of sufficient purity for use in fuel cell cars.
Cars powered by proton exchange membrane (PEM) fuel cells depend on a supply of pure hydrogen. While some impurities (e.g. inert gases) are an issue in that their presence simply reduces the proportion of hydrogen present in the fuel and thus the efficiency of the car, other impurities have more serious consequences for the lifetime of the fuel cell. Cumulative and irreversible efficiency reduction through ‘poisoning’ of the catalyst (adsorption on to its surface, reducing the number of active sites), for example, is caused by even very small (ppm) amounts of sulphur.
Standards are under development which will specify hydrogen purity requirements at the refueling station. SAE J 2719, an international Standard which provides background information and a hydrogen fuel quality standard for commercial proton exchange membrane (PEM) fuel cell vehicles (‘Hydrogen fuel quality for fuel cell vehicles), serves as a starting point for ISO14687 (‘Hydrogen Fuel Product Specification for PEMFC Applications for Road Vehicles’), currently under development, as shown in the Table below. The specification from a commercial hydrogen provider (Linde) is also given for information. While the purity levels in the proposed standard are commensurate with an acceptable lifetime for fuel cells, clearly lower levels of impurities would be even more advantageous.
Contaminantupper limits inppm unless spec-SAEISO DISLindeLimit ofified otherwiseJ 271914687-25.0detectionHydrogen fuel99.99%99.97%100.00%—indexTotal100300——allowable non-H2, non-HeWater———Liquid water5551Water vapourTotal220.50.05hydrocarbons(C1 basis)Oxygen5520.1InertsHeliumTBD300—10NitrogenTBD10030.1Argon—Carbon Dioxide110.1Carbon0.20.20.1MonoxideTotal sulphur0.0040.004—0.1compoundsH2S———0.005Formaldehyde0.010.01—0.06Formic Acid0.20.2—0.2Ammonia0.10.1—0.1Total0.050.05—0.005-0.05halogenatedcompoundsParticulate10 μm ——0.1(max. size)Particulate1 μg/l1 mg/kg—0.005 mg/kg;concentration1 μg/l
Furthermore, it may be financially advantageous to reduce impurity levels. Calculations indicate that improving the CO levels in fuel from the ISO14687 proposed minimum of 0.2 ppm to 0.1 ppm, for example, may reduce the lifetime fuel cost by over 1%. This does not include potential reduced capital and maintenance costs which could also be exploited when purer hydrogen is used, resulting from use of lower catalyst loadings and higher current densities, combined with less frequent maintenance.